Originally Posted by dmksails
I've a question concerning the best way to estimate your total sailing time based not only on distance, but also considering wind speed and direction. (I'll ignore current for now)
For instance, My destination is 20 miles away, due north. Wind is also out of the north, so I'm not going to be sailing a straight course, but tacking to work my way north. Assume the average speed I'll make is 4 knots. How long will it take me to get to my destination and how did you determine this?
There are two numbers you need. Your angle to the TRUE wind, and your leeway.
For most monohull
cruising boats, the best angle to sail relative to the true wind is about 50 to 52 degrees, maybe a bit better, maybe a bit worse. Leeway can be a bit trickier to come by, but if you look at your instruments
and compare your heading to your course over ground (in the absence of current!) you'll come very close. Typically, for a deep draft
sailing in moderate winds, and calm seas, 5 degrees would be very optimistic, and 10 degrees very conservative. So let's call it 7 degrees.
So we are actually moving at an angle of ABOUT 58 degrees to the true wind. at 4 knots. With a little trigonometry we see that our speed straight upwind is equal to the cos(58 deg) * 4 knots. or (0.53 * 4) = 2.2 knots.
So your 20 mile trip straight upwind will take you 20/2.2=9.1 hours. It makes no real difference how many times you tack, the distance is the same.
Different boats will take different times. Catamarans typically can't sail as close to the wind, and make more leeway, but also go a bit faster.
Now, I promise, we are going to see a long list of people say "My boat does a LOT better than that!" and a few of them will be right. If you are sailing a FARR40 you might be sailing at 40 or even 37 degrees to the true wind, but that's not a typical cruising boat...